1. Field of the Invention
The present invention relates to systems for mechanically assisting the heart and more particularly to a drive system capable of controlling the rise time and plateau level of a pressure pulse applied to a cardiac compression apparatus.
2. Discussion of the Related Art
Heart disease accounts for one of the leading causes of cardiac dysfunction among individuals worldwide. One of the more common heart disorders often associated with heart disease involves substantial weakening of the heart. Left unaided, a critically weakened heart often cannot pump the necessary blood required to sustain bodily functions.
An important life-saving technique for individuals diagnosed with weakened hearts includes mechanically assisting the heart to pump blood. Such support ensures an adequate blood pressure for sufficiently supplying blood throughout the body without undue stress on the heart muscle. Typically, a device such as a heart compression apparatus carries out the assistance during invasive surgery. An alternative device incorporating cardiopulmonary resuscitation (CPR) techniques externally compresses the chest to rhythmically squeeze the heart area and assist in increasing blood flow.
Those skilled in the art have proposed a variety of devices to successfully carry out the heart compression function to maximize support for the heart and provide reliable and accurate functionality. One such device, disclosed in pending Provisional U.S. patent spplication Ser. No. 60/028,722, filed on Oct. 18, 1996 and assigned to the assignee of the present invention, carries and supports the heart during invasive surgery while uniformly applying pressure directly to the heart through means of an inflatable liner. The liner is cyclically inflated and deflated by an inflation system to apply pressure to the heart.
Because each heart pumps blood according to a pressure profile unique for each patient, successful cardiac compression on the inflatable liner depends upon the inflation system being controllable to somewhat match the patient's personal cardiac rhythm or pressure profile. An important consideration is the sharp rise in ventricular pressure during the early portion of systole, which provides only about fifty to one hundred milliseconds within which to establish synchronous compression.
One conventional inflation system for controlling the rise time and plateau level of a pressure pulse utilizes a single regulator-reservoir configuration. The system includes a compressor connected to a regulator, and a reservoir disposed downstream of the regulator. An outflow valve connects to the reservoir outlet and is placed in fluid communication with an inflation chamber disposed in a cardiac compression apparatus for supporting and assisting a heart. During operation, the compressor supplies flow through the regulator, which maintains a desired pressure in the tank. Inflation of the chamber occurs by opening the outflow valve to produce an exponentially increasing pressure transient within the chamber that asymptotically approaches the supply pressure level to define a relatively constant pressure, or plateau level, within a known response time.
While this system works relatively well to ensure that the pressure applied to the liner never exceeds the supply pressure, the configuration provides a relatively accurate adjustment of the plateau level without any independent control over changes in the rise time of the start transient. Generally, the waveforms may be expressed as: EQU P=P.sub.PLATEAU *1-exp(-t/T)!
wherein t represents time, and T represents the system time constant. As illustrated in FIG. 3, regardless of the plateau setting, the time constant T remains unchanged with changes only in the magnitude of the plateau level. Because of this characteristic, control of the rise time using the conventional inflation system is generally possible only by affecting the system resistances, capacitances, or source regulator pressure. As a result, the response of the transient to reach the plateau level is typically of the order of three time constants. The rise time, known commonly as the time required to reach the plateau level, generally depends upon system characteristics such as overall resistance and capacitance. Therefore, having the capability of merely controlling the plateau level does not enable independent control over the rise time.
Moreover, because of the controlled conditions associated with surgical environments, the conventional inflation system is often disposed several feet from the inflation chamber. As a result, a relatively long hose generally couples the inflation system to the compression apparatus liner. Coupled with the effects of overall system resistances and pressure load, the resulting time constant realized by the conventional inflation system substantially degrades system performance. Alternatively, should the resistance be lowered through expansion of the tubing diameter, the size, weight and cost for the components required to effect a larger flowrate would render the system undesirable from a practical standpoint.
A second proposal solves several of the aforementioned problems by implementing a source pressure substantially higher than the desired plateau pressure. The pressure circuit is similar to the first proposal described above, but includes an additional control valve disposed in series with the outflow valve. Operation of the system depends upon precise timing to initially open both valves simultaneously, exposing the liner to an exponentially increasing pressure characteristic of the source pressure level, then closing the control valve upon reaching a desired plateau pressure and trapping the pressure within the liner for a timed duration.
While this proposal is somewhat beneficial in offering a way of controlling the rise time independently of the desired plateau pressure, the precise timing required to effectively control the plateau level by closing the control valve at precisely the right instant is difficult in practice to achieve. This is typically due to the equilibrium time required from the instant the valve closes until the plateau pressure is actually realized. As a result, the system often exhibits an undesirable overshoot or undershoot of the plateau pressure causing an unexpected deviation in the cardiac compression apparatus liner.
Therefore, those skilled in the art have recognized the need for an inflation system capable of following a predefined pressure profile with independent control of both rise time and plateau level with a minimum number of components and accurate repeatability. The inflation system of the present invention satisfies these needs.